EP 0124842 describes a "Power plant with a combustor with a fluidized bed". The combustor burns the fuel supplied to the fluidized bed. The plant is provided with means for adjusting the bed level in dependence on the operating conditions by the transfer of bed material from the combustor to a storage vessel and vice versa, wherein the storage vessel for bed material is connected to the combustor by means of discharge and reinjection conduits and the storage vessel is connected, via a conduit provided with a valve, to a space with a pressure lower than that in the combustor, and wherein the reinjection conduit is connected, via a conduit provided with a valve, to a pressure gas source, the pressure of which is higher than the pressure in the combustor. Both the combustor and storage vessel of the plant are enclosed within a pressure vessel.
Otherwise, the prior art regarding means for handling bed material in plants with a combustor with a fluidized bed is described in detail in a report ANL/CEN/FE-81-3 from Argonne National Laboratory, Argonne, Ill., entitled "Discharge and Handling of Solids from Pressurized Fluidized Bed Combustors" by John E. Hanway Jr. and W. F. Podolski.
A PFBC plant comprises a large number of more or less interdependent control systems with various forms of external limitations. An important control system in this connection is the control which is designed to keep the bed at a constant temperature of about 850 degrees centrigrade independently of the output of power from the plant.
There is a relatively clear-cut relationship between the available power output from the plant and the bed level. To be able to maintain a constant power output, there must be a continuous supply of the coal mixture which is burnt. The ashes which are then generated tend to increase the amount of bed material in the bed, whereby the bed level tends to rise. The fact that this does not take place is due to the fact that a bed level control with a set value corresponding to the desired and current bed level ensures that an amount of bed material is discharged which is equal to the production of new bed material during combustion.
When a load change occurs, the bed level needs to be changed to be adapted to the new load condition. Since the production of new bed material in spite of all is relatively modest, the rate of increase which would be obtained in the bed if the discharge of bed material were stopped is normally far from sufficient to provide the desired speed of the bed level change in case of an increased load requirement. It is true that, in case of a reduction of load, the speed of the bed material discharge could be increased; however, such a method alone could not generally take care of load changes. As is clear from the above, in the method disclosed in the EP patent the bed level is adjusted upon a load change by injecting more bed material from the storage vessel in case of increased power requirement in order thus to raise the bed level, and that bed material is sucked out from the bed to be able to lower the bed level.
The above is, per se, a simple, relatively fast and safe method for adjusting the bed level upon a load change. However, this method imposes very high demands on the temperature control of the bed, in fact so high demands that it is a factor limiting the rate at which load changes can take place. The reason for this is that, although the storage vessel of the bed material is located inside the pressure vessel, the bed material stored in the storage vessel has a considerably lower temperature than the bed. To reduce the disturbing influence on the temperature control which is caused by this injection and discharge of bed material, it would be desirable if the bed material in the storage vessel had a temperature which largely corresponds to the bed temperature. A heating of the bed material may, of course, be performed in a plurality of different ways. The present invention describes an alternative way of heating the bed material in the storage vessel while utilizing the heat storage constituted by the bed itself.